High performance spin-valve transistor

ABSTRACT

The invention generally relates to the field of spintronics, a branch of electronics using the magnetic spin properties of electrons. More particularly, the invention relates to the field of spin-valve transistors which can be used in numerous fields of electronics. The invention aims to propose an original arrangement for producing high-level and high-contrast collector currents simultaneously. The inventive spintronics transistor comprises a semiconductor emitter, a base fanning a spin valve and a metallic collector separated from the base by an insulating deposit. The emitter/base interface constitutes a Schottky barrier and the base/collector interface constitutes a tunnel-effect barrier.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present Application is based on International Application No.PCT/EP2003/050886, filed on Nov. 24, 2003, which in turn corresponds toFR 02/15845 filed on Dec. 13, 2002, and priority is hereby claimed under35 USC §119 based on these applications. Each of these applications arehereby incorporated by reference in their entirety into the presentapplication.

1. Field of the Invention

The invention generally relates to the field of spintronics, a branch ofelectronics using the magnetic spin properties of electrons. Moreparticularly, the invention relates to the field of spin-valvetransistors which can be used in numerous fields of electronics, eitheras an individual component (logic gate, non-volatile memory element,etc.), or as a magneto-resistive sensor in numerous fields (automotive,instrumentation, drilling and navigation), or even as a read headsupporting high-capacity magnetic recording (recording densities greaterthan a terabit/inch²).

2. Description of the Prior Art

In a ferromagnetic body, the diffusion of the electrons differsaccording to their spin. This effect is used in magnetic multilayerdevices which are also called spin-valve devices for creating a giantmagnetoresistance effect.

Its principle is represented in FIGS. 1 a and 1 c. A spin valvecomprises three successive layers of materials. The first layer F1 is alayer of ferromagnetic metal with imposed magnetization. The secondlayer N is a layer of non-magnetic metal separating the first layer fromthe third. The third layer F2 is a layer of ferromagnetic metal withvariable magnetization. The operating principle is as follows: if thespin valve is subjected to a magnetic field H, the latter is sufficientto modify the direction of magnetization of the second layer withoutaffecting that of the first layer. The imposed magnetization of thesecond layer F2 then remains after the magnetic field H has beenremoved. Depending on the direction of the magnetic field applied, thefirst and the third layer are then either in a parallel configuration(FIG. 1 a), with both magnetizations pointing in the same direction(black vertical arrows in FIG. 1 a), or in anti-parallel configuration(FIG. 1 c), with the two magnetizations pointing in opposite directions(black vertical arrows in FIG. 1 c).

In the parallel configuration, the +½ spin electrons e⁻ (top obliquearrow in FIG. 1 a) pass through the layers F1 and F2 with a weakdiffusion. The resistances R+_(F1) and R+_(F2) of the layers F1 and F2therefore have a low value r for these electrons. The −½ spin electronse⁻ (bottom zig-zag arrow in FIG. 1A) pass through the layers F1 and F2with a strong diffusion. The resistances R−_(F1) and R−_(F2) of thelayers F1 and F2 therefore have a high value R for these electrons.Finally, the equivalent electrical resistance of the spin valve isrepresented in the diagram of FIG. 1 b. It is equivalent to two seriesresistors of valve r placed in parallel with two series resistors ofvalue R. If R is very high compared to r, the equivalent resistance ofthe circuit is approximately r.

In the anti-parallel configuration, the +½ spin electrons e⁻ (top arrowin FIG. 1 c) pass through the layer F1 with a weak diffusion(straight-line part of top arrow) and the layer F2 with a strongdiffusion (zig-zag part of top arrow). The resistance R+_(F1) of thelayer F1 therefore has a low value r for these electrons and theresistance R+_(F2) of the layer F2 has a high value R. The −½ spinelectrons e⁻ (bottom arrow of FIG. 1 c) pass through the layer F1 with astrong diffusion (zig-zag part of bottom arrow) and the layer F2 with aweak diffusion (straight-line part of bottom arrow). The resistanceR−_(F1) of the layer F1 therefore has a high value R for these electronsand the resistance R−_(F2) of the layer F2 has a low value r. Finally,the equivalent electrical resistance of the spin valve is represented inthe diagram of FIG. 1 d. It is equivalent to two resistors respectivelyof value r and R placed in parallel with two resistors also of values rand R. If R is very high compared to r, the equivalent resistance of thecircuit is now approximately R.

The value of the equivalent resistance of the spin valve is thusmodified according to the magnetic field applied.

One of the main areas of research in spintronics is in the developmentof spin-valve transistors. The spin-valve transistors offer majoradvantages over conventional semiconductor transistors such as, forexample, a low switching time, low energies involved and the possibilityof programming logic gates.

Various designs have been proposed since 1995. To illustrate thesedesigns represented in FIGS. 2, 3 and 4, a symbolic notation is used torepresent the different layers of the transistor. The symbols used areas follows:

-   -   Layer F1 of ferromagnetic metal with permanent magnetization:        rectangle with a single arrow.    -   Layer F2 of ferromagnetic metal with variable magnetization        dependent on the magnetic field: rectangle with two arrows        head-to-tail.    -   Layer N of non-magnetic metal: empty rectangle.    -   Semiconductor layer presenting an electronic Schottky barrier:        rectangle topped by a spiked curve, symbolizing the Schottky        barrier.    -   Insulating layer I: lozenge with vertical sides.

The vertical disposition of the different layers is representative ofthe potential differences applied. Two layers situated at differentheights are therefore subject to a potential difference. V_(EB) is usedto denote the potential difference existing between the emitter and thebase and V_(BC) the potential difference existing between the base andthe collector.

In 1995, a first concept was proposed (D. J. Monsma, J. C. Lodder, T. J.A. Popma and B. Dieny—Perpendicular Hot Electron Spin-Valve Effect in aNew Magnetic Field Sensor: The Spin-Valve Transistor—Physical reviewLetters—Vol. 74, No. 26, Jun. 1995). This concept is represented in FIG.2. The proposed transistor comprises an emitter E of semiconductormaterial, a metallic base B with three layers F1, N and F2 forming aspin valve and a collector C also of semiconductor material. Theemitter/base and base/collector junctions are of Schottky type asindicated in FIG. 2. The arrow indicates the direction of the collectedcurrent. It is opposite to the direction of electron propagation.Electrons are injected from the emitter to the base through the base.Some of these electrons, called hot electrons, have sufficiently highenergy to pass through the emitter/base Schottky junction. The energyrelaxation of these hot electrons in the metallic base depends on theirspin. The collected current I_(C) strongly depends on the relativeorientation of the magnetizations between the layers F1 and F2. The termmagneto-current contrast MC is used to describe the following ratio:MC=(I _(C,P) −I _(C,AP))/(I _(C,P) +I _(C,AP))with I_(C,P) being the maximum current transmitted when themagnetizations are in parallel configuration and I_(C,AP) the minimumcurrent transmitted when the magnetizations are in anti-parallelconfiguration.

Strong collector current I_(C) contrasts have been observed with such adevice (P. S. A. Kumar et al., Physica C350, 166 (2001)).

However, the relaxation effects of the electrons in the base aresignificant, the latter comprising a number of successive interfaces,and, on the other hand, the energy of the electrons depends on thepotential barrier level difference between the two emitter/base andbase/collector Schottky junctions. Now, it is technologically verydifficult to produce significant Schottky junction level differences(greater than 1 eV). Thus, this device can generate only very low-levelcollector currents, of around 10 nA.

In 2001, a second spin-valve transistor concept was proposed (S. vanDijken, Xin Jiang, and S. S. P. Parkin—room temperature operation of ahigh output current magnetic tunnel transistor—Applied Physics.Letters—Vol. 80, No. 18-6 May 2002). This so-called MTT (for MagneticTunnel Transistor) transistor is represented in FIG. 3. It comprises anemitter consisting of a ferromagnetic layer F1 with permanentmagnetization, an insulator I, a base B consisting of a ferromagneticlayer F2 with variable magnetization and a collector C of semiconductormaterial. The base/collector junction is of Schottky type as isindicated in FIG. 3. The potential differences V_(EB) and V_(BC)required between the base and the emitter and the base and the collectorare also represented. The spin-polarized electrons are emitted from theferromagnetic emitter E by tunnel effect in the ferromagnetic base B.The MTT can be used to limit the relaxation effects of the electrons inthe base which is now formed by only a single layer. Higher-levelcurrents I_(C) at the output of the collector are then obtained.However, the magnetic tunnel junction configuration leads to lowercontrasts in current I_(C) between parallel and anti-parallelmagnetization configurations (less than 70%). This results from the factthat this device does not exploit the spin-dependency of thecharacteristic relaxation length of the hot electrons.

Finally, in 2002, a variant of the MTT was proposed (S. S. P.Parkin—Intermag Europe Conference—Amsterdam—May 2002). This isrepresented in FIG. 4. It comprises an emitter E of semiconductormaterial, an insulator I, a base B which is a spin valve comprisingthree metallic layers F1, N and F2 and a collector of semiconductormaterial. The base/collector junction is of Schottky type. The emitteremits, by tunnel effect, non-spin-polarized electrons towards thespin-valve structure of the base B. Very high collector currentcontrasts (greater than 3000%) have been observed with this structure.However, the voltage V_(EB) that can be applied between the emitter andthe base is limited by the breakdown phenomenon in the tunnel barrierand consequently limits the intensity of the emitter current I_(E). Thelevel of the collector current I_(C) which is proportional to the levelof the emitter current I_(E) also remains limited.

SUMMARY OF THE INVENTION

The object of the invention is to provide a new spin-valve transistorarrangement with which to produce both a high-level and high-contrastcollector current I_(C), which is desirable for sensor type applications(weak field detectors or read heads) or as non-volatile memory elementor even as programmable logic gate.

More specifically, the subject of the invention is a spin-valvetransistor comprising an emitter, a base and a collector, the emitterbeing made of a semiconductor material, the base comprising threesuccessive metal layers, the first layer and the third layer beingferromagnetic, the second layer not being ferromagnetic, the interfacebetween the emitter and the layers of the base forming a Schottky diode,characterized in that the collector is metallic and separated from thebase by a thin insulating layer of approximately a few nanometers, saidlayer forming a tunnel-effect barrier between the base and saidcollector.

Advantageously, the insulating layer presents a lower-level potentialbarrier than the potential barrier of the Schottky diode existingbetween the emitter and the base.

Advantageously, said insulating layer is made of tantalum oxide or ofzinc sulfide or of zirconium oxide or of a rare earth oxide such asyttrium oxide.

Advantageously, the insulating layer has a thickness of approximatelybetween 1 and 4 nanometers.

Advantageously, the emitter comprises at least one layer ofsemiconductor material and the collector at least a first layer ofmetallic material, the layer of semiconductor material of the emittercomprises at least a second layer of metallic material for connectingelectrical connection means. These electrical connection means areimplanted on the level of the first layer of metallic material, on thelevel of the second layer of metallic material and of any one of thelayers of the base, said connection means being used to apply externalvoltages and currents to the transistor.

Finally, the electrical voltage applied between the emitter and the basevia the connection means is advantageously greater than the potentialbarrier of the insulating layer.

Still other objects and advantages of the present invention will becomereadily apparent to those skilled in the art from the following detaileddescription, wherein the preferred embodiments of the invention areshown and described, simply by way of illustration of the best modecontemplated of carrying out the invention. As will be realized, theinvention is capable of other and different embodiments, and its severaldetails are capable of modifications in various obvious respects, allwithout departing from the invention. Accordingly, the drawings anddescription thereof are to be regarded as illustrative in nature, andnot as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood, and other advantages willbecome apparent, from reading the description which follows, given as anonlimiting example and with reference to the appended figures in which:

FIGS. 1 a, 1 b, 1 c and 1 d represent schematic diagrams of a spin valveand equivalent circuit diagrams in the parallel and anti-parallelstates.

FIG. 2 represents the symbolic diagram of a first embodiment of aspin-valve transistor according to the prior art.

FIG. 3 represents the symbolic diagram of an MTT-type spin-valvetransistor according to the prior art according to a first variant.

FIG. 4 represents the symbolic diagram of an MTT-type spin-valvetransistor according to the prior art according to a second variant.

FIG. 5 represents the symbolic diagram of a spin-valve transistoraccording to the invention.

FIG. 6 represents the arrangement of the various layers of saidtransistor and the associated electrical connections of the transistoraccording to the invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 5 represents a symbolic diagram of the spin-valve transistoraccording to the invention. It comprises an emitter E of semiconductormaterial, a metallic base B made up of three layers F1, N and F2 forminga spin valve, an insulating material I and a collector C of electricallyconductive material. The emitter/base junction is of Schottky type asindicated in FIG. 5. The arrow indicates the direction of the collectedcurrent. Electrons are injected from the emitter to the base through theemitter/base Schottky junction. The electrons pass from the base B tothe collector C through the insulator I either by tunnel effect orballistically. This arrangement has two major advantages over the priorarrangements. The use of a Schottky type emitter/base junction allowshigher emitter/base voltages V_(EB) to be used, no longer limited by thebreakdown phenomenon. It is thus possible to obtain high emittercurrents I_(E) and, consequently, high collector currents I_(C). Sincethe base is formed by a spin valve, the MC contrast of the collectorcurrent can also assume high values.

To optimize the device, the materials must be chosen to obtain both ahigh-level Schottky barrier and a low-level tunnel barrier, lower thanthe Schottky barrier level. The insulator can in particular be made oftantalum oxide or zinc sulfide or of zirconium oxide or of a rare earthoxide such as yttrium oxide. The material of the emitter isconventionally a semiconductor material such as silicon or galliumarsenide. The material layers forming the base are, in particular,cobalt or a cobalt alloy for the ferromagnetic layer F1, copper or goldfor the neutral layer N, a nickel and iron alloy such as permalloy (with80% nickel) for the ferromagnetic layer F2, and finally the conductivelayer can be of copper or gold.

The collector current I_(C) is the sum of two currents: I_(tunnel),tunnel current between base and collector, and I_(ballistic), ballisticcurrent from the emitter made up of the electrons having sufficientenergy to pass through the Schottky junction and then the base withoutrelaxing. Since the tunnel current serves no purpose in the operation ofthe transistor, it should be minimized. In conventional electronics, itcorresponds to a leakage current. The simplest means is to thicken theinsulator I used as a tunnel barrier between the base and the collector,the tunnel current decreasing exponentially with this thickness.

It is also advantageous to use an emitter/base voltage V_(EB) greaterthan the level of the tunnel barrier. In this case, a significantportion of the electrons can pass over the tunnel barrier to reach thecollector ballistically. Thus, the level of the collector current isincreased.

Technologically, the spintronic transistor according to the invention ispresented as a stack of layers as represented in FIG. 6. This stack canbe produced by deposition methods used in conventional microelectronics.It comprises, successively, a metallic layer A, the semiconductor layerof the emitter E, the three metallic layers F1, N and F2 forming thebase, the layer of insulating material I and the metallic layer C of thecollector. The electrical connection of the emitter, the base and thecollector are provided by connection means C_(E), C_(B) and C_(C) whichcan, for example, be metallic terminals. These connection means C_(E),C_(B) and C_(C) are located on the level of the metallic layer A locatedunder the emitter E, on the level of the base and on the layer C of thecollector. The connection can be made at the level of the base on anyone of the three layers F1, N or F2. FIG. 6 also shows an electricalpolarization diagram of the transistor. A current generator linked tothe transistor by the connection means C_(E) and C_(B) imposes a currentI_(E) on the input of the emitter and a voltage V_(EB) between theemitter and the base. A voltage generator linked to the transistor bythe connection means C_(C) and C_(B) imposes a voltage V_(BC) betweenthe base and the collector. The current collected by the collectordepends on the configuration of the magnetizations imposed on theferromagnetic layers of the base.

It will be readily seen by one of ordinary skill in the art that thepresent invention fulfills all of the objects set forth above. Afterreading the foregoing specification, one of ordinary skill will be ableto affect various changes, substitutions of equivalents and variousother aspects of the invention as broadly disclosed herein. It istherefore intended that the protection granted hereon be limited only bythe definition contained in the appended claims and equivalents thereof.

1. A spin-valve transistor comprising: an emitter, a base, and acollector, wherein the emitter being made of a semiconductor material,the base comprising three successive metal layers, the first layer andthe third layer being ferromagnetic, the second layer not beingferromagnetic, the interface between the emitter and the layers of thebase forming a Schottky diode, wherein the collector is metallic andseparated from the base by a thin insulating layer of approximately afew nanometers, said layer forming a tunnel-effect barrier between thebase and said collector, wherein the electrical voltage applied betweenthe emitter and the base via the connection means and is greater thanthe potential barrier of the insulating layer, wherein a collectorcurrent is the sum and a tunnel current between the base and thecollector means and the ballistic current from the emitter and thecollector current has sufficient energy to pass through the base and theSchottky diode without relaxing.
 2. The spin-valve transistor as claimedin claim 1, wherein the insulating layer presents a lower-levelpotential barrier than the potential barrier of the Schottky diodeexisting between the emitter and the base.
 3. The spin-valve transistoras claimed in claim 2, wherein the insulating layer is made of tantalumoxide or of zinc sulfide or of zirconium oxide or of a rare earth oxidesuch as yttrium oxide.
 4. The spin-valve transistor as claimed in claim1, wherein the insulating layer has a thickness of approximately between1 and 4 nanometers.
 5. The spin-valve transistor as claimed in claim 4,wherein the layer of semiconductor material of the emitter comprises atleast a second layer of metallic material.
 6. The spin-valve transistoras claimed in claim 4, comprising: electrical connection means connectedto the emitter, base, and collector layers and are placed on top on thelevel of the first layer of metallic material, on the level of thesecond layer of metallic material and of any one of the layers of thebase, said connection means being used to apply external voltages andcurrents to the transistor.
 7. The spin-valve transistor as claimed inclaim 1, wherein the emitter comprises at least one layer ofsemiconductor material and the collector at least a first layer ofmetallic material.
 8. The spin-valve transistor as claimed in claim 7,comprising: electrical connection means is connected to the emitter,base, and collector layers and placed on top on the level of the firstlayer of metallic material, on the level of the second layer of metallicmaterial and of any one of the layers of the base, said connection meansbeing used to apply external voltages and currents to the transistor.